This invention relates to numerically controlled machine tools, and more particularly to an automatic measuring method for such a machine tool and a device for practicing the method.
A variety of methods and devices for measuring the outside diameter, the inside diameter, the length, etc. of a workpiece have been proposed in the art. The present applicant has proposed a novel automatic measuring device under Japanese Patent Application No. 61212/1973. This device will now be described with reference to FIG. 11.
A detecting head 2 is provided on the cross slide 1 of a numerically controlled turret lathe. The detecting head 2 can be set aside by rotating it from the measuring position A for measuring to the waiting position B for machining. The cross slide 1 are moved by pulse motors 4 and 5 which is driven by numerical information inputted into a numerical control device 3 so that two differential transformer type sensors 6 and 7 provided on the detecting head 2 in the measuring position A are allowed to alternately come in contact with two points P.sub.1 and P.sub.2 which are opposite to each other along a diameter to be measured of a workpiece 9 held by a chunk 8 at the end of the main spindle of the lathe, thereby to detect the deviations of the workpiece from the desired dimension. These deviations are inputted into a calculation unit 11 through an analog-to-digital converter 10 thereby to obtain the sum of the deviations and the mean value of the deviations, which are converted into tool position compensating data for the tip of the tool 13 which is secured to the turret 12, the data being applied to the numerical control device 3.
As is apparent from the above description, in the automatic measuring device, the two styluses, i.e., the differential transformer type sensors 6 and 7 are alternately brought into contact with the two points P.sub.1 and P.sub.2 opposite to each other along the relevant diameter of the workpiece, whereby the mean value of the deviations between the desired value and the actual value obtained by the styluses 6 and 7 is employed as the tool position compensating data. Therefore, with the automatic measuring device described above, the measurement can be achieved independently of the displacement of the main spindle due to the thermal displacement of the machine tool, and also the non-coincidence of the machine coordinates with the program coordinates.
However, the automatic measuring device still suffers from the following disadvantages: Since the positioning accuracy on numerical control of the cross slide 1 is employed as the reference in measurement, the positioning accuracy, i.e., the measurement accuracy is liable to be affected by heat generation of the pulse motors 4 and 5 adapted to move the cross slide 1 and by the zigzag motion of the cross slide. Furthermore, as the detecting head 2 employs the differential transformer type sensors 6 and 7, the detecting head is elastically deformed by its contact pressure applied to the workpiece. In addition, it is impossible for the automatic measuring device to measure the diameter of a relatively small hole.